In locomotives having electric motors, it has become common practice to use asynchronous motors. One or more motors are powered by an inverter providing three-phase AC at the required frequency and itself powered from a DC source, with the DC optionally being obtained by rectification.
The required frequency is determined from the speed of rotation of the motor. When it is greater than the frequency which corresponds to the speed of rotation of the motor, slip occurs between the rotor and the flux in the motor, thereby generating torque which tends to accelerate rotation of the rotor. When the frequency is no greater than the frequency which corresponds to the speed of rotation of the motor, then the motor is braked.
When the locomotive is running at high speed, motor control by means of a torque-controlling handle therefore consists essentially in adjusting the frequency generated by the inverter as a function of the difference observed between the requested torque and the motor torque, and this difference is evaluated on the basis of the current and the voltage supplied to the motor. The voltage supplied to the motor, referred to as "fullwave voltage" is then constant, and its peak value is equal to 2/.pi. times the DC voltage of the source. The inverter operates merely as a reversing switch at the frequency set by a frequency control.
At low speed, the voltage must be reduced in order to control the flux in the motor so as to minimize losses in the motor and in the inverter. Motor control then requires not only the frequency to be adjusted as mentioned above, but also the AC voltage supplied to the motor to be adjusted. The inverter thus receives, in addition to its frequency control signal, a control signal for modulating the voltage to be supplied, and in response to these two control signals, it operates in chopper mode. The inverter control circuit generates width-modulated pulses which, when integrated, produce a sinewave voltage at the required frequency and at the requested reduced amplitude.
In practice, the inverter itself is essentially constituted by power switches and is separate from its control circuit which receives and processes the above-mentioned control signals and delivers trigger signals to the power switches. Given that its main function is to produce trigger signals for width-modulated pulses, this control circuit is referred to below as a pulse modulator.
The invention relates more particularly to the control system which generates the frequency control signals together with the voltage modulus or voltage magnitude control signals for delivery to said pulse modulator under low-speed conditions.
Such control systems are known. A discussion on this topic is to be found in the article "Adjustable speed AC drives--a technology status review" by B. M. Bose, published in the journal "Proceedings of the IEEE", Vol. 70, No. 2, Feb. 1982. Generally, such a control system generates a frequency control signal from the torque control signal, with the requested torque being compared with the torque developed by the motor as determined by measuring the motor voltages and currents, in order to provide a torque error signal which is then processed in order to derive a frequency difference signal which is then added to a frequency representing rotation of the rotor in order to provide a frequency control signal. Simultaneously, the flux in the motor is observed relative to a constant reference flux value, and is used to determine the voltage control signal. Such a circuit comprising two independent control loops is neither accurate nor stable and it is incapable of getting close to optimum efficiency.
The object of the present invention is to provide a vector control system for a squirrel-cage asynchronous electric motor that enables better efficiency and higher stability to be obtained.